Fix some segfaults

internal/system/box.go

@@ -69,6 +69,7 @@ func (this *System) newBox (outer anyBox) *box {
 		system:  this,
 		outer:   outer,
 		drawer:  outer,
+		tags:    make(util.Set[string]),
 	}
 	box.canvas = util.NewMemo (func () canvas.Canvas {
 		if box.parent == nil { return nil }
@@ -614,7 +615,8 @@ func (this *box) propagateAlt (callback func (anyBox) bool) bool {
 func (this *box) transparent () bool {
 	// TODO uncomment once we have
 	// a way to detect texture transparency
-	return util.Transparent(this.attrColor.Value()) /*&&
+	col := this.attrColor.Value().Color
+	return col == nil || util.Transparent(col) /*&&
 		(this.texture == nil || !this.texture.Opaque())*/
 }
 

internal/system/style.go

@@ -11,7 +11,7 @@ type styleApplicator struct {
 func (this *styleApplicator) apply (box anyBox) {
 	if box.Role() != this.role {
 		// the role has changed, so re-cache the list of rules
-		this.rules = make([]*tomo.Rule, 4)
+		this.rules = make([]*tomo.Rule, 0, 4)
 		for _, rule := range this.style.Rules {
 			role := box.Role()
 			// blank fields match anything
@@ -27,10 +27,12 @@ func (this *styleApplicator) apply (box anyBox) {
 	attrs := make(tomo.AttrSet)
 	for _, rule := range this.rules {
 		satisifed := true
-		for _, tag := range rule.Tags {
-			if !box.Tag(tag) {
-				satisifed = false
-				break
+		if rule.Tags != nil {
+			for _, tag := range rule.Tags {
+				if !box.Tag(tag) {
+					satisifed = false
+					break
+				}
 			}
 		}